Novel high-resolution data to unravel distribution, mechanisms and anthropogenic threats to freshwater macrophytes worldwide

Freshwater macrophytes are fundamental to the health of inland waters, portraying unique eco-physiological adaptive strategies that make them distinctive from terrestrial vascular plants. We integrated a novel, high-resolution global distributional dataset of 3382 freshwater macrophyte species (representing 97% of all known freshwater macrophytes worldwide), encompassing 486,252 occurrence records across more than 12,700 1° grid cells to investigate species richness patterns and their underlying mechanisms across the world. Freshwater macrophyte richness shows a hump-shaped pattern globally, peaking at approximately 40° N in temperate regions and at 20° S in tropical to subtropical latitudinal bands. Widespread species contributed the most to the northern peak while other species contributed to the southern peak. Coastlines and adjacent lowland regions are the macrophyte diversity hotspot globally. Contemporary temperature emerged as the dominant environmental gradient structuring global macrophyte richness across and within major continents, especially for widespread species, often displaying a quadratic relationship. However, our range-size-dependent analysis revealed a critical divergence: geometric constraints imposed by the mid-domain effect are the pre-eminent spatial drivers for intermediate and restricted-range species. Model explanatory power varied significantly across continents and decreased sharply with species range size. The overall positive correlation between macrophyte richness and the Human Influence Index is driven entirely by widespread generalist species. This relationship vanishes for restricted and highly restricted species, demonstrating that macrophyte endemism centres are uniquely vulnerable. Synthesis. The high-resolution, range-size-dependent analysis resolved critical ambiguities in macrophyte macroecology. We establish a robust framework for understanding that the drivers of freshwater plant diversity are strongly hierarchical and scale-dependent, thereby laying the foundation for urgent spatially targeted conservation efforts focused on protecting vulnerable centres/regions of macrophyte endemism.